The present invention relates to xe2x80x9cmicrowave assisted rapid and economical process for the preparation of substituted phenylaldehydes from trans and toxic cis-phenylpropenes: a commercial utilization of toxic cis-isomerxe2x80x9d in which industrially important phenylaldehydes (e.g. asaronaldehyde where R1 is CHO, R2xe2x95x90R4xe2x95x90R5 is xe2x80x94OMe and R3xe2x95x90R6 is H; p-anisaldehyde where, R1 is xe2x80x94CHO, R2xe2x95x90R3xe2x95x90R5xe2x95x90R6 is H and R4 is xe2x80x94OMe and vetralaldehyde where R1 is xe2x80x94CHO, R2xe2x95x90R5xe2x95x90R6 is H; R3xe2x95x90R4 is xe2x80x94OMe or the like) of the formula I 
are obtained via oxidation of easily available isomeric forms (trans and cis-isomer) of (R2xe2x80x94R3xe2x80x94R4xe2x80x94R5xe2x80x94R6)phenylpropene bearing essential oils (i.e. xcex2-asarone, anethole, and methyl isoeugenol or the like) wherein R2 to R6 equal or different, being hydrogen or hydroxy or acyl or alkyl or methylenedioxy or alkoxy groups or the like, under microwave irradiation using meta-periodate/osmium tetraoxide (catalytic amount) for a reaction time less than 20 minutes in biphasic system comprising a solvent and aqueous phase containing a catalyst (such as quaternary ammonium salt and amberlite IRA-410 etc) with high yield varying from 71-82% depending upon the reagent, reaction time, condition and the phenylpropene used. In addition crude Acorus calamus oil (rich in xcex2-asarone present in 70-94%) used directly for microwave assisted oxidation is an added benefit as remaining constituents of calamus oil do not interfere for the preparation of asaronaldehyde (yield just less by 5-10% depending upon asarone percentage in calamus oil) which makes the above process further cost effective since tetraploid and hexaploid varieties of Acorus calamus has been internationally banned for their use in human consumption. Moreover, we have observed that the preparation of asaronaldehyde (a versatile drugs precursor) requires lesser time (2-20 minutes) under microwave irradiation while oxidation takes 2-6 hours when conducted at room temperature (conventional method).
Naturally occurring substituted phenylaldehydes (Harborne, J. B. and Baxter, H., In: Phytochemical Dictionary, A Handbook of Bioactive Compounds from Plants, Taylor and Francis Ltd., London WC1N 2ET, 472-488 (1993)) e.g. vanillin, p-anisaldehyde, p-hydroxybenzaldehyde, asaronaldehyde, heliotropin and vetralaldehyde etc possess in common an aromatic ring bearing one or more hydroxy or dioxymethylene or alkoxy groups or the like, attached to a aldehyde group (CHO) contribute significantly to the taste and flavour of many foods, drinks, perfumery and serve as a pharmaceutical aid. In addition, phenylaldehyde derivatives serve as a raw material for the preparation of a large number of aromatic compounds useful in the perfume industry e.g. treatment of phenylaldehyde with alkali alcoholate results in the formation of phenyl benzoate and condensation of phenylaldehyde derivative with acetaldehyde gives cinnamic aldehyde which are useful in both the perfume and pharmaceutical industries. In addition, large quantities of phenylaldehydes are used in the manufacture of dyes, medicines (Patel, P. J.; Messer Jr., W. S. and Hudson, R. A., J. Med. Chem., 36, 1893-1901 (1993)), photographic films, cosmetics, dyes, agrochemicals etc.
The widespread aromatic aldehydes such as vanillin is obtained from the pods of Vanilla planifolia (family: Orchidaceae), the bulbs of Dahlia spp. (Compositae), the sprouts of Asparagus spp. (Liliaceae), the beats of Beta spp. (Chenopodiaceae) and also from the essential oils of Syzygium aromaticum (Myrtaceae), Ruta spp. (Rutaceae), Spiraea spp. (Rosaceae) and Gymnadenia spp. (Orchidaceae), while 3,4-methylenedioxybenzaldehyde (heliotropin) is obtained from the essential oils of the flowers and leaves of Robinia pseudacacia (Legumminosae), Doryphora sassafras, Eryngium potericum (Umbelliferae), Heliotropium spp. (Boraginaceae), Vanilla spp. (Orchidaceae) and from extracts of Viola spp. (Violaceae) and Baccharis rosmarinifolia (Compositae). Other phenylaldehydes are restricted to a few families such as p-anisaldehyde occurs in the fruits of Pelea madagascariensis (Rutaceae), Agastache rugosa (Labiatae), leaves of Magnolia salicifolia (Magnoliaceae) and also in the essential oils of Vanilla spp. (Orchidaceae), Acacia spp. (Leguminosae), Cassia spp. (Leguminosae), Pinus spp. (Pinaceae), Pimpinella anisum (Umbelliferae), Illicium verum (Illiciaceae), whereas p-hydroxybenzaldehyde occurs in traces in Plocama pendula (Rubiaceae), Pterocarpus marsupium (Leguminosae), and asaronaldehyde in the essential oils of Acorus spp. (Motley, T. J., Economic Botany, 48: 397-412, (1994) and Piper spp. (Koul, S. K., Taneja, S. C., Malhotra, S. and Dhar, K. L., Phytochemistry, 32(2): 478-480, (1993)). However, the limited percentage of these substituted phenylaldehydes present in the plant kingdom is not sufficient to fulfill the world demand and as a result, the major amounts of phenylaldehydes are made synthetically.
A number of processes have been proposed to prepare substituted phenylaldehydes such as p-anisaldehyde, dimethoxybenzaldehyde, vanillin, heliotropin, asaronaldehyde etc. For the most part, these methods involves reacting the substituted benzene, such as p-methoxybenzene, 1,2,4-trimethoxybenzene with freshly distilled phosphorus oxychloride (POCl3) in the presence of anhydrous N,N-dimethylformamide (DMF). However, while this Vilsmeier-Haack method has been proven to be useful, they suffer from one or more process deficiencies. For example, some processes of this type necessarily involve resort to sub ambient temperatures, which, of course, involves some considerable process control. In addition, large excesses of DMF and POCl3 must necessarily be employed to carry out the synthesis to obtain appreciable yields and moreover, POCl3 give rise to a violent exothermic reaction leading to obvious problems. Lastly, in some cases, the reaction is effected by the formation of some side reaction products (Toril, S., Uneyama, K. and Ueda, K., J. Org. Chem., 49, 1830-1832 (1984).
Typical prior art refrences include U.S. Pat. Nos. 2,794,813; 5,358,861; 3,799,940; European Patent No. EP-A 405,197; Japanese Pat. Nos. 10,754,442A2; 55,87, 739; British Pat. Nos. 417,072; 774,608; 1,092,615; U.S.S.R. Pat. No. 490,793 and German Pat. Nos. 57,808; 207,702.
It therefore becomes an object of the invention to provide rapid and economical process for the preparation of substituted phenylaldehydes from trans and cis-phenylpropenes which further provide commercial utilization of toxic cis-isomer as well as eliminate the above discussed disadvantages and others.
The main object of the present invention is to develop a rapid and economical process for the preparation of useful phenylaldehydes (such as p-methoxybenzaldehyde, vetralaldehyde, asaronaldehyde etc) in one step.
Another object of the invention is to develop a simple process for the preparation of phenylaldehyde in high purity.
Another object of the invention is to develop a simple process for the preparation of phenylaldehyde with minimum or no side product formation such as corresponding acid.
Yet another object of the invention is to develop an easy work-up of the reaction product.
Yet another object of the invention is to develop a simple process for high degree of conversion.
Yet another object of the invention is to develop a process, which does not require anhydrous reaction medium, a condition preferred by industries.
Yet another object of the invention is to develop a simple process which does not require explosive and expensive reagents, hence, capable of undergoing commercial scale production.
Yet another object of the invention is to develop a simple and quick process for the preparation of substituted phenylaldehydes in a short time ranging from 2 to 20 minutes under microwave irradiation.
Yet another object of the invention is to develop a process for the preparation of value added products from toxic compound (such as xcex2-asarone).
Yet another object of the present invention is to explore the possibilities of preparing important aldehydes utilizing otherwise toxic essential oil e.g. crude calamus oil of tetraploid or hexaploid varieties or the other essential oil rich in anethole, isosafrole, methyl isoeugenol (at least above 75% in crude oil) or the like, thereby, enhancing the profitable use thereof.
Yet another object of the present invention is to explore a simple and cheaper starting material in which what ever percentage of cis (toxic) and trans-isomer (non-toxic) exists in crude essential oil or formed during alkaline isomerisation of xcex3-phenylpropenes (such as methylchavicol, safrole, methyl eugenol etc) are capable of undergoing oxidation into high valued phenylaldehydes otherwise the percentage of cis-isomer higher than a limited amount is not allowed with trans-isomer for commercial use in perfumery, flavour and pharmaceutics.
In brief, the present invention provides microwave assisted rapid (reaction time less than 20 minutes) and economical process for the preparation of substituted phenylaldehydes from trans and cis-phenylpropene derivatives using meta-periodate and osmium tetraoxide (catalytic amount) as an efficient oxidizing agent in the presence of catalyst namely amberlite IRA-410 and quaternary ammonium salt. It is worthwhile to mention that the conversion of toxic xcex2-asarone (cis-isomer) from Acorus calamus or xcex2-asarone (70-94%) rich crude calamus oil directly into asaronaldehyde (a versatile drugs precursor) is an economical gain of the above invention since it provides a proper utilization of internationally banned tetraploid and hexaploid varieties derived essential oil of Acorus calamus. 
Accordingly, the present invention provides a microwave assisted rapid and economical process for the preparation of substituted phenylaldehydes from trans and toxic cis-phenylpropenes, a commercial utilization of toxic cis-isomer of the general formula (I), 
wherein R1 is xe2x80x94CHO,
R2, R3, R4, R5, R6 are independently selected from
i) a hydrogen atom;
ii) an alkoxy group at least two of R2, R3, R4, R5, R6 being hydrogen atom; or an alkoxy group but one of R2, R3, R4, R5, R6 being methylenedioxy group in combination with either hydroxyl group, an alkoxy group, an alkyl group having at least one carbon atom, an aryl group and a hydrogen atom or an alkoxy group but one of R2, R3, R4, R5, R6 being hydroxyl group in combination with either methylenedioxy group, a hydroxyl group, an alkoxy group, an alkyl group having at least one carbon atom, an aryl group (xe2x80x94C6H5)or a hydrogen atom;
iv) a hydroxyl group at least one of R2, R3, R4, R5, R6 being a hydrogen atom in combination with either an alkoxy group, a hydroxyl group, a methylenedioxy group, an alkyl group having at least one carbon atom, an aryl group or a hydrogen atom;
v) a protected hydroxyl group such as acetyl, benzyl, etc at least one of R2, R3, R4, R5, R6 being a hydrogen atom in combination with either an alkoxy, a hydroxyl group, a methylenedioxy group, an alkyl group having one or more carbon atoms, an aryl group or a hydrogen atom; said substituted phenylaldehydes being from corresponding (R2xe2x80x94R3xe2x80x94R4xe2x80x94R5xe2x80x94R6) phenylpropene derivatives, said process comprising oxidizing phenylpropene derivatives in presence of an oxidizing agent and optionally a co catalyst in a solvent at a mole ratio of 1:1 to 1:12 for a period ranging from 20 seconds to 20 minutes under microwave radiation, removing the solvent under reduced pressure and isolating the product in a conventional manner to obtain a yield between 71 to 82% of substituted phenylaldehydes.
In an embodiment, the solvent used is selected from the group consisting of ether solvent such as tetrahydrofuran, dimethyoxyethane, dioxane; ketonic solvents selected from acetone, ethylmethyl ketone; alcohols selected from methanol, ethanol and in presence of water.
In another embodiment of the invention, the oxidizing agent used is selected from potassium permanganate/base, manganese dioxide/sulphanilic acid, meta-periodate/osmium tetraoxide.
In still another embodiment of the invention, the co catalyst used is selected from amberlite such as amberlite-410, quaternary ammonium salt such as benzyltriethyl ammonium chloride, base such as triethylamine, pyridine.
In yet another embodiment of the invention, the mole ratio of phenylpropene derivatives to oxidizing agent is ranging from 1:1 to 1:6.
In yet another embodiment of the invention, the radiation frequency of microwave used is ranging from 2000 to 2800 MHz.
In yet another embodiment of the invention, the starting material phenylpropene is widely available natural phenylpropanoid.
In yet another embodiment, both isomeric forms (E and Z) of phenylpropene are utilized for phenylaldehyde formation.
In yet another embodiment of the invention, toxic cis-isomer is converted into value added natural aldehyde.
In yet another embodiment, an internationally banned xcex2-asarone from Acorus calamus is utilized by its conversion into a useful asaronaldehyde.
In yet another embodiment, the above process is capable of preparing phenylaldehyde derivatives on commercial scale.
In yet another embodiment of the invention, the above process oxidizes crude calamus oil of tetraploid or hexaploid varieties or the other essential oil rich in anethole, isosafrole, dimethoxy isoeugenol (at least above 75% in crude oil).
In yet another embodiment of the invention provides a process wherein, the phenylaldehyde derivatives in highest purity without any contamination of corresponding acid and alcohol.
In yet another embodiment, the above process provides phenylaldehyde derivatives in a very short time period ranging from 2-20 minutes.
In yet another embodiment of the invention, the above process allows conducting reaction in aqueous medium, a condition preferred by industries and provides easy work-up of the reaction product.
In yet another embodiment provides a simple process and cheaper starting material, in which what ever percentage of cis (toxic) and trans-isomer (preferred) exists in crude essential oil or formed during alkaline isomerisation of xcex3-phenylpropenes (such as methylchavicol, safrole, methyl eugenol etc) are oxidized into high valued phenylaldehydes, otherwise higher than the allowed percentage of cis-isomer formed along with the trans-isomer is not allowed for commercial use in perfumery, flavour and pharmaceutics.
In yet another embodiment of the invention provides a process wherein, in the above process for the preparation of some new phenylaldehydes, which are useful as a simple starting material for synthesis of corresponding acids, esters, amides, alcohol, and xcex2-unsaturated aldehyde and are also useful for the dyes, alkaloids, agrochemical etc.
Broadly speaking the invention relates to cis and trans-isomeric forms of (R2xe2x80x94R3xe2x80x94R4xe2x80x94R5xe2x80x94R6)phenylpropene bearing essential oils such as xcex2-asarone, anethole and methyl isoeugenol or the like wherein R2 to R6 equal or different, being hydrogen or hydroxy or acyl or alkyl or methylenedioxy or alkoxy groups or the like, are oxidized under microwave irradiation using meta-periodate/osmium tetraoxide for a reaction time less than 20 minutes in biphasic system comprising a solvent and aqueous phase containing a catalyst and thus high valued industrially important substituted phenylaldehydes such as asaronaldehyde, p-anisaldehyde, vetralaldehyde or the like derivatives are obtained in a single step in high yield varies from 71-82% depending upon the reagent, reaction time, condition and the phenylpropene used, the conversion of toxic xcex2-asarone (cis-isomer) from Acorus calamus or xcex2-asarone (70-94%) rich crude calamus oil directly into asaronaldehyde (a versatile drugs precursor) is an economical gain of the above invention since well explored Acorus calamus (tetraploid and hexaploid varieties) has recently been banned internationally for their use in human consumption.
Accordingly, the present invention provides microwave assisted rapid and economical process for the preparation of substituted phenylaldehydes from trans and toxic cis-phenylpropenes of Formula I, a commercial utilization of toxic cis-isomer wherein R1 is fixed as a xe2x80x94CHO, however, R2, R3, R4, R5, R6 are independently; i) a hydrogen atom; ii) a alkoxy group but at least two of them from R2, R3, R4, R5, R6 are hydrogen atom or a alkoxy group but one methylenedioxy group with combination of either hydroxyl group, alkoxy group, alkyl group having at least one carbon atom, aryl group (xe2x80x94C6H5) and hydrogen atom or a alkoxy group but one hydroxyl group with combination of either methylenedioxy group, hydroxyl group, alkoxy group, alkyl group having at least one carbon atom, aryl group and hydrogen atom; iii) a methylenedioxy with at least three of them from R2, R3, R4, R5, R6 are combination of either alkoxy, hydroxy group, alkyl group having at least one carbon atom, aryl group and hydrogen atom; vi) a hydroxyl group but one of them from R2, R3, R4, R5, R6 is hydrogen atom with combination of either alkoxy, hydroxyl group, methylenedioxy group, alkyl group having one carbon atom, aryl group and hydrogen atom; vii) a protected hydroxyl group such as acetyl, benzyl, etc but at least one of them from R2, R3, R4, R5, R6 is hydrogen atom with combination of either alkoxy, hydroxyl group, methylenedioxy group, alkyl group having one or more carbon atom, aryl group and hydrogen atom or the like, obtained from corresponding (R2xe2x80x94R3xe2x80x94R4xe2x80x94R5xe2x80x94R6)phenylpropene derivatives (e.g. anethole where R2xe2x95x90R3xe2x95x90R5xe2x95x90R6xe2x95x90H; R4xe2x95x90OMe; methyl isoeugenol where R2xe2x95x90R5xe2x95x90R5xe2x95x90H; R3xe2x95x90R4xe2x95x90OMe and b-asarone where R2xe2x95x90R4xe2x95x90R5xe2x80x94xe2x95x90OMe; R3xe2x95x90R6xe2x95x90H etc) and the above process comprising the steps of (a) providing phenylpropene such as but not limited to 2,4,5-trimethoxyphenylpropene (b-asarone) in the following solvents namely ether such as but not limited to tetrahydrofuran, dimethoxyethane, dioxane, and the like; ketone such as but not limited to acetone, ethylmethyl ketone; alcohol such as but not limited to methanol, ethanol and the like and water; (b) oxidation of phenyipropene derivatives in above solution by adsorbing on oxidizing reagents such as but not limited to meta-periodate/osmium tetraoxide (catalytic amount) and the like to be used in the ratio of 1-12 moles, preferably 1-6 moles per mole of phenyipropene derivative in a short period ranging from 2-20 minutes under microwave irradiation; (c) oxidation step proceeds more smoothly along with higher yield in presence of co-catalyst amberlite such as but not limited to amberlite-410, quaternary animonium salt such as but not limited to benzyltriethylanimonium chloride or base such as but not limited triethylamine, pyridine; (d) filtering the mixture and removing the solvent under reduced pressure, where the product is to be isolated by a conventional manner, i.e. extraction, recrystallization and chromatography and the yield of the product (e.g. 2,4,5-trimethoxybenzaldehyde where R1xe2x95x90xe2x80x94CHO, R2xe2x95x90R4xe2x95x90R5xe2x80x94xe2x95x90OMe; R3xe2x95x90R6xe2x95x90H; 4-methoxybenzaldehyde where R1xe2x95x90xe2x80x94CHO, R2xe2x95x90R3xe2x95x90R5xe2x95x90R6xe2x95x90H; R4xe2x80x94xe2x95x90OMe and 3,4-dimethoxybenzaldehyde where R1xe2x95x90xe2x80x94CHO, R2xe2x95x90R5xe2x95x90R6xe2x95x90H; R3xe2x95x90R4xe2x80x94xe2x95x90OMe etc in the above formula I) varies from 68-81% preferably more in case of meta-periodate/osmium tetraoxide as a oxidizing reagent.
In one more embodiment of the present invention, a simple and cheaper starting material phenylpropene is utilized for high valued phenylaldehyde derivatives, and one step process is described for substituted phenylaldehyde in high purity and yield without contamination of corresponding acid and alcohol.
In another embodiment of the present invention, a simple and quick process for the preparation of substituted phenylaldehydes in a short time ranging from a few seconds to a few minutes under microwave irradiation.
In another embodiment of the present invention, a process for the preparation of value added products from toxic compound (such as xcex2-asarone).
In another embodiment of the present invention, a simple and cheaper starting material in which what ever percentage of cis (toxic) and trans-isomer (non-toxic) exists in crude essential oil or formed during alkaline isomerisation of xcex3-phenylpropenes (such as methylchavicol, safrole, methyl eugenol etc) are capable of undergoing oxidation into high valued phenylaldehydes otherwise the percentage of cis-isomer higher than a limited amount is not allowed with trans-isomer for commercial use in perfumery, flavour and pharmaceutics.
Plant cells are highly sophisticated chemical factories where a large variety of chemical compounds are synthesized with great precision and ease from simple raw materials at normal pressure and temperature. Beside foods, plant materials/chemicals are used for many purposes for example, for treating medical ailments, dyeing clothes, colouring food items and for perfumery, cosmetics, flavour etc. Flavours represent a growing demand within the food industry. Several methods including chemical synthesis, biotechnology and natural extraction are under progress for the smooth production of aroma chemicals. Some of aromatic phenylaldehydes, mainly produced in plants in response to pathogen attack, possess strong antimicrobial activity due to hydroxy and an aldehyde group attached to the aromatic ring of phenylaldehyde. Therefore, as per applications concern, these phenylaldehydes are not only widely used in fragrances, flavours, cosmetics, liquors, pharmaceuticals but they are also utilized as antibacterials, antifungals, and as biologically active compounds. Moreover, phenylpropenes, produced by plants in high concentration (sometimes unto 95-96% ) are also widely used by perfumery, flavour and pharmaceutical industries, e.g. anethole (4-methoxyphenylpropene) is well exploited essential oil which exists in cis- and trans-form (Miraldi, E.; Flavour and Fragrance Journal, 14(6) 379-382 (1999)), but its corresponding phenylaldehydes have more demand, as only trans-anethol is allowed since cis-anethol is possibly considered toxic and hazardous to human beings. On the other side, vanillin (a phenylaldehyde) is one of the most commonly consumed flavour chemicals (5,550 t/a worldwide) (Somogyi, L. P., Chem. Ind. L., 5, 170, (1996). However, the limited percentage and high price of these natural phenylaldehyde led to the necessity of using large amount of synthetic materials. At present, 97% of the world vanilla flavour market is synthetic vanillin and remaining 3% (weight basis) is a natural vanilla extract (Taylor, A. J. and Mottram, D. S., In: Flavour Science, Recent Developments, The Royal Society of Chemistry, Thomas Graham House, Science Park, Milton Road, Cambridge CB4 4WF, UK, 111-137, (1996). Various processes are known for the preparation of phenylaldehydes (Schiraldi, D. A. and Kenvin, J. C., U.S. Pat. No. 5,910,613; and Soma, Y., JP Pat. No. 11049734A2; Kashima, M., Yoshimoto, H., Noda, Y. and Jibiki, H. JP Pat. No. 7330655A2; Kajisori, S., JP Pat. No. 2268130A2; Tanaka, M., Sakakura, T., Wada, H. and Sasaki, Y. JP Pat. No. 3264546A2; Kawamoto, K., Yoshioka, T., Yamagata, H., JP Pat. No. 5087739A2 and Ito, N. and Hasebe, A. JP Pat. No. 11279104A2) using phenyipropenes as a starting material. Considering the cost of starting material and reagents, phenylpropenes are found best suitable and cost effective starting material for the synthesis of substituted phenylaldehydes such as vanillin from isoeugenol, p-methoxybenzaldehyde from anethole and heliotropin from isosafarole (U.S. Pat. Nos, 1,643,804; 2,794,813). However, there is so far no such industrial method available for the preparation of a versatile drug intermediate xe2x80x9c2,4,5-trimethoxybenzaldehydexe2x80x9d (a phenylaldehyde) from b-asarone (a toxic phenyipropene). It is worthwhile to mention that the selection of b-asarone for the preparation of asaronaldehyde has several fold benefits such as a simple and cheaper starting material and a proper utilization of internationally banned toxic calamus oil. Since b-asarone has recently been proved to be toxic and carcinogenic (Taylor, J. M., Jones, W. I., Hogan, E. C., Gross, M. A., David, D. A. and Cook, E. L., Toxicol. Appl. Pharmacol., 10: 405 (1967); Keller, K.; Odenthal, K. P. and Leng, P. E., Planta Medica, 1: 6-9 (1985) and Kim, S.C., Liem, A., Stewart, B. C. and Miller, J. A., Carcinogensis, 20(7), 1303-1307 (1999)) and the most affected plant is Acorus calamus (family:Araceae) in which percentage of toxic b-asarone depends upon the varieties of A. calamus (Riaz, M., Shadab, Q., Chaudhary, F. M., Hamdard Medicus 38(2): 50-62 (1995) and MeGuffin, M., Hobbs, C., Upton, R. and Goldberg, A., In: American Herbal Products Association""s Botanical Safety Handbook, CRC Press, Inc.; Boca Raton, Fla.; USA, 231, (1997)). The content of b-asarone in the triploid variety is 8-19%, while b-asarone reaches upto 96% in the tetraploid and hexaploid varieties (extensively found in Asian countries). In contrast, b-asarone is not found in the diploid variety. As a result, the calamus oil obtained from North American diploid strain (zero, b-asarone) and East European triploid strain (unto 12% b-asarone) are allowed for clinical effectiveness and safety while the calamus oil produced in Asian belt (such as India, Pakistan, Bangladesh, Nepal, Japan and China) has diminished the market potential of calamus oil due to high percentage of b-asarone ranging from 70 to 94% (Mazza, G., J. of Chromatography 328:179-206 (1985); Nigam, M. C., Ateeque, A., Misra, L. N. and Abmad, A., Indian Perfumer 34: 282-285 (1990) and Bonaccorsi, I., Cortroneo, A., Chowdhury, J. U. and Yusuf, M., Essenze Derv. Agrum, 67(4): 392-402 (1997)). Therefore, our objective to utilize toxic b-asarone for value added phenylaldehyde does not provide only economical gain for calamus oil of tetraploid or hexaploid strain but also as a simple and cheaper starting material for the preparation of a natural 2,4,5-trimethoxyphenylbenzaldehyde (asarylaldehyde), a versatile drug intermediate for synthesis of several biologically active compounds (Abmad, S., Wagner, H. and Razaq, S., Tetrahedron, 34 (10): 1593-1594, (1978)) including Makaluvamine-D and Discorhabdin-C marine alkaloids (Sadanandan, E. V., Pillai, S. K., Lakshmikantham, M. V., Billimoria, A. D., Culpepper, J. S. and Cava, M. P. J. Org. Chem., 66: 1800-1805, (1995)).
Phenylpropenes are relatively electron rich (pi bonds) which can be oxidized to corresponding aldehyde with a number of oxidizing agents such as chromic acid (Ger. Pat. No. 576 and U.S. Pat. No. 2,794, 813), manganese dioxide (Br. Pat. No. 774,608), potassium permanganate (Erlenmeyer, Chem. Ber. 9, 273 (1876)), chromyl chloride (U.S. Pat. No. 365,918), air (Ger. Pat. No. 224,071), oxygen (Ger. Pat. No. 150,981), ozonolysis (Ger. Pat. No. 321,567 and C.A., 54,:5538 (1960)), electrolysis (Ger. Pat. No. 92,007), peroxide (Ger. Pat. No. 93,938), nitrobenzene (Brit. Pat. No. 271,819 and U.S. Pat. No. 1,643,804), nitrobenzene (Brit. Pat. No. 285,156) and by several others process.
All the above methods have various limitations, for example, low yield, expensive reagents and formation of unwanted side products. Our initial efforts to oxidize xcex2-asarone by using known oxidizing reagents such as potassium permanganate or manganese dioxide/sulphanilic acid (Br. Pat. No. 774,608)) coupled in microwave to obtain asaronaldehyde is remained unsuccessful due to poor yield with various side product formation (such as asaronic acid). Fortunately, the combination of osmium tetroxide (OsO4) and periodate reagent (Cainelli, G., Contento, M., Manescalchi, F. and Plessi, L, Synthesis, 47-48, (1989)) in microwave is found an effective and high yielding method for preparing asaronaldehyde from toxic xcex2-asarone (Example Ia). In addition, microwave assisted oxidation of phenylpropenes or the like with osmium tetroxide conducting in fuming hood is easy and safe otherwise handling of osmium tetraoxide requires special precaution due to its poisonous nature. After success of xcex2-asarone, oxidation of xcex1-asarone into asaronaldehyde is also found as effective as xcex2-asarone. These experiments gave us idea that geometry (i.e. cis/trans-isomer) of phenylpropene does not effect the yield of final oxidized product. In continuation of this oxidation, calamus oil (rich in phenylpropene i.e. 85-90% xcex1/xcex2-asarone) is also capable of undergoing oxidation without any problem in purification or loss in yield since other constituents of calamus oil does effect interfere in asaronaldehyde formation (Example 1c). Although, OsO4/NaIO4 is well known system for converting alkene into aldehyde, however, microwave assisted oxidation of phenylpropene has not been reported so far for the preparation of substituted phenylaldehydes especially from toxic isomer of phenylpropene i.e. xcex2-asarone (cis-isomer) or phenylpropene rich crude oil (i.e. crude calamus oil). Having successfully achieved an efficient process for asaronaldehyde, we decided to extend the process to convert various phenylpropenes into phenylaldehyde via OsO4/NaIO4 such as anethole into 3,4-dimethoxyisoeugenol into 3,4-dimethoxybenzaldehyde (Example II), 4-methoxybenzaldehyde (Example III) or the like. It is further worthwhile to mention that what ever percentage of cis/trans anethole from methyl chavicol, cis/trans isosafrole from safrole, cis/trans isoeugenol from eugenol or the like obtained during alkaline isomerisation of xcex3-phenylpropene (allylbenzene) or crude essential oil rich cis/trans phenylpropene (above 70% for industrial scale) can be easily utilized for the formation of corresponding phenylaldehydes. Our present invention is also beneficial to those industries which are engaged in alkaline isomerisation of allylbenzene into trans-phenylpropene for wide scope in flavour, perfumery and pharmaceutical industries, however, formation of cis-phenylpropene along with trans-phenylpropene diminished their applications, since separation of isomers is tedious and expensive on industrial point of view whereas, both isomeric forms can be used for the preparation of high valued phenylaldehydes. 